When a company acquires another company, business unit, or product line, the purchasing company may also want to acquire corresponding IT assets from the seller. Specifically, the buyer may want to acquire some or all of the domain controllers that host the user accounts, computer accounts, or security groups that correspond to the business assets that are to be purchased. The only supported methods for the buyer to acquire the IT assets that are stored in the seller's Active Directory forest are as follows:
1. Acquire the only instance of the forest, including all domain controllers and directory data in the seller's entire forest.

2. Migrate the needed directory data from the seller’s forest or domains to one or more of the buyer's domains. The target for such a migration may be an entirely new forest or one or more existing domains that are already deployed in the buyer's forest. We recommend that you migrate the directory data without security identifier (SID) history. If you migrate the directory data with SID history, information about the seller's forest will be retained in the new forest of the buyer. For more information about migrating directory data without SID history, see Migrating Accounts Without Using SID History (http://go.microsoft.com/fwlink/?LinkId=113694).

This support limitation exists because:
• Each domain in an Active Directory forest is assigned a unique identity during the creation of the forest. Copying domain controllers from an original domain to a cloned domain compromises the security of both the domains and the forest. Threats to the original domain and the cloned domain include the following

◦ Sharing of passwords that can be used to gain access to resources

◦ Insight regarding privileged user accounts and groups

◦ Mapping of IP addresses to computer names

◦ Additions, deletions, and modifications of directory information if domain controllers in a cloned domain ever establish network connectivity with domain controllers from the original domain

• Cloned domains share a common security identity; therefore, trust relationships cannot be established between them, even if one or both of the domains have been renamed.
This means that prune and graft between forests or to create new forest is not supported

Microsoft will not support the division of Active Directory forests into two or more different forests. You cannot take a domain out of one forest and put it into its own forest or move it between forests and expect to have Microsoft support you. The reasons for this are stated in the article above, I just wanted to use this section to restate the different ways that people have asked this question, so that more people can find the answer.

You can use domain migration ADMT to move accounts between domains and forests, if you need to move users, groups, computers, and other resources around. That is supported.

Also, in plain English, you don't want to break off parts of your forest because, if you do, you:
1.compromise the security of the all the new forests you create
2.make future interconnection impossible and create a situation were troubleshooting is unnecessarily complex. Essentially, someone in the future could waste a lot of time trying to figure out that you created two or more forests in an unsupported scenario that can never be reconnected - even via trust relationships.